The present invention relates to current sources and, more particularly, to precision integrated transistor current sources.
Transistor current sources are widely used in integrated circuits as load devices, current scaling and bias elements. Such current sources can result in insensitivity of circuit performance to both power supply and temperature variations. Current sources are often more economical than resistors requiring less die area.
The most simplest prior art transistor current source consists of a reference current that is sourced to the collector of a first, diode-connected transistor. The base of this transistor, besides being connected to its collector, is connected to the base of a second transistor with both transistors having their respective emitters returned to ground reference. Since the two transistors have the same base-emitter voltage, the collector current of the second transistor is a function of the collector current of the first transistor as is understood. For example, by matching the emitter areas of the two transistors the collector currents will be substantially equal.
The above example ignores both the "Early" voltage effect as well as errors due to base current of the latter of which is a function of the forward current gain, beta, of the transistors. In typical bipolar linear integrated processes the betas of the transistors are quite high such that any errors due to base current can be neglected. The same is true for the "Early" voltage effect. However, in some present day low voltage digital integrated processes transistor betas can be very low when compared to those of the linear processes. In addition, the "Early" voltage effects can not be neglected. For instance, the ratio of the reference current to the current flowing in the second or output transistor can be as bad as 0.88 for base-emitter voltages of 0.7 volts. This is very undesirable.
Hence, a need exists for a low voltage precision current source that can be fabricated in integrated circuit form suing present day low voltage digital integrated processes.